1. Field of the Invention
The present invention relates to particle beam irradiation systems and, more particularly, to a charged particle irradiation system that treats a tumor or other affected part by irradiating the affected part with a charged particle beam.
2. Description of the Related Art
In a known therapy method, a cancer or other affected part of a patient is irradiated with a charged particle beam (ion beam) of, for example, either a proton or carbon ion beam. A charged particle irradiation system used in such a therapy typically includes an ion beam generator, a beam transport line, and an irradiation room. An ion beam generated by the ion beam generator is transported to the irradiation room by the beam transport line and reaches an irradiation nozzle in the irradiation room. The ion beam that reaches the irradiation nozzle is thin. The irradiation nozzle includes a scanning magnet that scans the ion beam traveling therethrough. In order to form a uniform dose distribution over an irradiation target, such as a cancer tumor, the scanning magnet of the irradiation nozzle can irradiate an entire irradiation target with the ion beam that is scanned in a direction (lateral direction) perpendicular to a beam axis. Irradiating the irradiation target with this ion beam forms a dose distribution that has a peak at a specific depth as determined by energy of the ion beam. The peak of the dose distribution is called a bragg peak. Because the bragg peak has a narrow spread of a few mm in a depth direction, ion beam irradiations with varying energies achieve uniform irradiation of the target.
A method of widening the dose distribution called a scanning irradiation method to which the present invention pertains uses a scanning magnet to scan the ion beam to thereby widen the dose distribution in the lateral direction and change the energy to thereby widen the dose distribution in the depth direction. The scanning irradiation method offers a benefit of an efficient use of the ion beam thanks to a small energy loss of the ion beam in the irradiation nozzle. Due to the beam scanning and energy change involved in the method, however, the affected part or other irradiation target is irradiated so as to be sequentially painted. If the irradiation object is displaced during irradiation, for example, if the affected part is displaced as the patient breathes, a uniform dose distribution may not be formed.
JP-2008-154627-A discloses a method for performing repainting irradiation. In this method, a displacement waveform representing a displacement amount of the irradiation object is obtained in advance and, only when the irradiation object is in a predetermined phase during irradiation, a dose is divided and delivered in a plurality of times. If the displacement waveform changes radically from what was obtained in advance, the method stops irradiation and resumes irradiation at the same phase in the next cycle to thereby form a desired dose distribution.
JP-2006-288875-A discloses a charged particle irradiation system that is controlled such that the scanning magnet disposed in the irradiation nozzle bends the ion beam in a direction perpendicular to a direction in which the ion beam travels and irradiation is repeated in units of a spiral cycle. The charged particle irradiation system is controlled such that, upon repainting irradiation, the scanning magnet controls an irradiation position of the ion beam in units of a spiral cycle so as not to allow irradiation of the ion beam to be interrupted in the middle of the spiral cycle, thereby achieving a uniform dose distribution.
In the charged particle irradiation system based on the scanning irradiation method according to the present invention, means is provided for forming a uniform dose distribution in a moving irradiation target.
According to the method disclosed in JP-2008-154627-A, irradiation time may be longer by irradiation interruption, if the displacement waveform of the irradiation object is not stable.